Zoom-in on spectral windows of the CHIANTI output that were considered for our analyses. We note the logarithmic scale on the ordinate and the numerous contributing lines. Without loss of generality, the bubble used is that shown in the top panels of Fig. 2. The hydrogen-rich PN case has been chosen in order to highlight the N VII line, which is virtually absent for the WR composition. We show the spectra at different resolutions (see legend), the spectral range and resolution chosen for the line integration (red), the continuum (short-dashed), and the domains where the continuum is defined for the integrations (blue, on the abscissae).